This invention relates to a humidity sensor, whose sensing element comprises a dielectric metal oxide layer and exhibits a change in its electrostatic capacitance with changes in humidity in the surrounding atmosphere.
Measurement of humidity with high precision is still difficult compared with the measurement of other physical variables of the atmosphere such as temperature and pressure. However, the need of easy and accurate measurement of humidity is increasing in food industries, in the field of agriculture and in many other fields either to control humidity or to accommodate something to humidity.
As a general trend, preference has been given to electrical methods of measuring humidity. One of now prevailing methods of providing an electrical indication of humidity is the use of a deliquescent salt, such as lithium chloride, which undergoes a change in its ionic conductivity with a change in its moisture content, and another is the use of a hygroscopic substance, such as magnetite or a silicon semiconductor, which exhibits a change in electrical resistance as it absorbs and desorbs moisture.
However, humidity sensors as embodiments of these methods, i.e. humidity sensors utilizing ionic conductivity, exhibit considerable drifting of the indications with the passage of time by reason of polarization, and, besides, electrical indications of these sensors have dependence also on the adsorption of various gaseous substances other than moisture. Furthermore, these humidity sensors are not fully satisfactory in their responsiveness, magnitude of hysteresis and the width of humidity range they can cover.
Conventional electric humidity sensors are not limited to the above described types. A humidity sensor comprising a filament of an organic material such as human hair, nylon or polystyrene and a strain gauge to detect the deformation of the filament caused by absorption and desorption of moisture is also popular. However, this type of sensor is unsatisfactory in its accuracy, responsiveness and magnitude of hysteresis. A still different type of humidity sensor utilizes the swelling of a synthetic resin containing fine particles of an electrically conducting material such as carbon or a metal, but this type of sensor is low in sensitivity and easily deteriorates by the influence of moisture. A further example of known methods is the use of a porous layer of alumina for detecting a change in humidity as a change in the capacitance of the alumina layer resulting from the adsorption of moisture in the pores and desorption therefrom, but this method has a drawback in showing a great drifting of the indications over a relatively short period.
Highly accurate measurement of humidity is possible by means of an apparatus on the principle of .alpha.-ray absorption and transmission, but this apparatus is too large-scaled and too costly to be of general use.
Thus, humidity sensing devices and apparatus now on the market or under development all have certain drawbacks in their functional characteristics, price and/or convenience for usage and maintenance: none of them is fully satisfactory in every respect.